[analyzer] Do not step into statements while collecting function decls.
[analyzer] Do not step into statements while collecting function decls.

CallGraph's recursive visitor only needs to collect declarations; their
bodies will be processed later on. RecursiveASTVisitor will recurse on
the bodies if the definition is provided along with declaration.
Optimize, by not recursing on any of the statements.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@158934 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang